This comparative ultrastructural study of the placenta focuses on areas in which trophoblast and maternal tissues are in closest contact. In all hemochorial placentas thus far studied, apparently viable fetal and maternal cells are separated by necrosis, fibrinoid, or both. In the basal plate of the macaque, the stromal reaction is much less extensive than in man. The cellular relations in the baboon's placenta are intermediate. Emphasis will be placed during the following year on studying other specialized cytotrophoblast and junctional areas: the chorion laeve, cell islands, cell columns, trophoblastic shell, and placental septa. The chorion laeve and decidua capsularis will be studied in particular detail to assess, by comparison with cellular relations in the basal plate, the influence of trophoblast in ultrastructural development of the decidua. The fetomaternal cellular relations will be studied in other less intimate forms of placentas in an effort to assess the significance of hypertrophic basal laminas, decidual "capsular" material, and submicroscopic sialomucin glycocalyces. The variations in endometrial ultrastructural response in normal human placentation and chorionic neoplasms will be studied. The mitochondrial and intranuclear specializations noted in the endometrial epithelium associated with hydatidiform mole will be studied morphologically in greater detail.